The Rockville second-grader didn't realize the Smurfberries she was buying on the popular game by Capcom Interactive were real purchases, much like buying a pair of shoes from Zappos or movie tickets from Fandango. After all, lots of children's games require virtual payments of pretend coins, treasure chests and gold to advance to levels.

But like a growing number of parents, Madison's mom, Stephanie Kay, was shocked to find very real charges from iTunes show up in her e-mail box days later.

"I thought the app preyed on children," she said. "Note that the Smurf app states it is for ages 4-plus."

The games are part of a category of applications on Apple's iTunes store that are free to download but let companies charge users for products and services when the application is launched. Following Apple, Google this week introduced these so-called "in-app purchases" for Android mobile phones and tablets, which experts say could create a new economy for newspapers, record labels and movie studios that have been struggling with ways to thrive online.

The in-app purchases have also catapulted children's games such as Smurfs' Village and Tap Zoo, by San Francisco-based Pocket Gems, into the ranks of the highest-grossing apps on iPods, iPhones and iPads.

But the practice is troubling parents and public interest groups, who say $99 for a wagon of Smurfberries or $19 for a bucket of snowflakes doesn't have any business in a children's game. Though a password is needed to make a purchase, critics say that the safeguards aren't strong enough and that there are loopholes.

"Parents need to know that the promotion of games and the delivery mechanism for them are deceptively cheap," said Jim Styer, president of Common Sense Media, a public advocacy group for online content for children. "But basically people are trying to make money off these apps, which is a huge problem, and only going to get bigger because mobile apps are the new platform for kids."

Apple said it tries to prevent episodes like Madison's by requesting a password when making in-app purchases. And parents can change settings on Apple's gadgets to restrict downloading and transactions, Apple spokeswoman Trudy Muller said.

But parents say changing those settings isn't easy or obvious.

Madison's mother let her download Smurfberries with the help of her older sister, who knew the family's iTunes password. From there, Madison went on a Smurfberry binge on the family iPad.

Arlington second-grader Leyla Ulku figured out her parents' password and recently racked up a $150 charge from buying buckets of stars and snowflakes to build a safari out of sea turtles and giraffes on Tap Zoo.